The fermentation of carbohydrates to produce biosynthetic water-soluble gums by the action of Xanthomonas bacteria is well known. The earliest work in this field was conducted by the U.S. Department of Agriculture and is described in U.S. Pat. No. 3,000,790. Particularly well known is the action of Xanthomonas campestris NRRL B-1459 on a glucose substrate.
Xanthomonas hydrophilic colloid (i.e., xanthan gum) is produced by transferring Xanthomonas campestris bacteria to a suitable medium and conditioning it to growth through two steps before allowing it to grow in a final medium containing 3 percent glucose. After 96 hours at 30.degree. C. with suitable aeration and stirring, Xanthomonas hydrophilic colloid is produced in approximately 1 percent concentration. Modified fermentation processes are described in U.S. Pat. Nos. 3,391,060; 3,391,061; 3,427,226; 3,455,786; 3,565,763; and the like.
Xanthomonas hydrophilic colloid is a microbial heteropolysaccharide which contains mannose, glucose, glucuronic acid, O-acetyl radicals and acetal-linked pyruvic acid in a molar ratio of 2:2:1:1:0.5.
While Xanthomonas campestris is the bacteria of choice for the purpose of producing the biosynthetic Xanthomonas hydrophilic colloid, other Xanthomonas species may be employed such as X. begoniae, X. malvacearum, X. carotae, X. incanae, X. phaseoli, X. vesicatoria, X. papavericola, X. translucens, X. vasculorum, and X. hedrae.
A typical raw fermentation broth is a viscous pseudoplastic solution containing between about 0.5-4 weight percent of microbial heteropolysaccharide, in addition to small amounts of salts, unreacted carbohydrate, Xanthomonas cells and other insoluble debris. The insoluble solids are difficult to remove from the heteropolysaccharide-containing broth, and their presence results in cloudy suspensions rather than clear solutions. For most applications, the presence of insoluble impurities in heteropolysaccharide-containing broths is undesirable. For example, such impurities tend to plug subterranean formations when an aqueous solution of a heteropolysaccharide is employed for oil recovery by water-flooding. Any attempt to clarify a heteropolysaccharide solution at an oil-well site is difficult and prohibitively expensive.
In a typical process for clarification of a Xanthomonas fermentation broth and/or recovery of the Xanthomonas hydrocolloid component, the broth is diluted with water to reduce its viscosity, and optionally the diluted broth is centrifuged or filtered to remove suspended insoluble solids. A salt such as potassium chloride and a nonsolvent such as methanol or isopropanol are added to the broth to flocculate the gum in the potassium form, which gum is then recovered by centrifugation or other solid/liquid separation technique. Further dissolution, reprecipitating and washing steps are usually employed.
U.S. Pat. No. 3,355,447 describes a process for treating a Xanthomonas hydrophilic colloid to improve its stability which involves heating an aqueous solution of the colloid at a temperature of 150.degree.-170.degree. F. for at least 20 minutes, cooling the heated solution to a temperature of 40.degree.-100.degree. F., adjusting the concentration of the medium to provide not more than about 1 percent by weight of colloid, and filtering the medium to recover a clarified hydrophilic colloid solution.
U.S. Pat. No. 3,516,983 describes a process for purifying a Xanthomonas hydrophilic colloid medium containing proteinaceous impurities which involves maintaining the pH of the aqueous medium above about 8, adding an alkali metal hypochlorite to the mixture, adjusting the pH of the medium to slightly acidic, and then adding a lower alcohol to precipitate the Xanthomonas hydrophilic colloid.
U.S. Pat. No. 3,591,578 describes an improved process for recovering a polysaccharide from a fermentation broth by precipitation, which involves heating the broth at a temperature of 80.degree.-130.degree. C. for a period of 10-110 minutes of a pH of 6.3-6.9 prior to precipitation.
U.S. Pat. No. 3,773,752 describes a method for recovering a microbial polysaccharide produced by Xanthomonas bacteria which involves diluting the fermentation broth with an alkaline metal salt solution to coagulate the insoluble solids which are present, and thereafter removing the coagulated solids by filtration.
U.S. Pat. No. 3,919,189 describes a method of decreasing the bacterial contamination of xanthan gum by treatment of the gum with propylene oxide gas.
U.S. Pat. No. 3,966,618 describes a method of clarifying a fermentation broth which contains dissolved xanthan gum and suspended solids which involves treating the broth with a minor amount of a protease enzyme. U.S. Pat. No. 4,010,071 and U.S. Pat. No. 4,119,491 also describe methods of broth clarification by means of protease enzyme treatment. U.S. Pat. No. 4,051,317 describes a method of precipitating xanthan gum from an aqueous solution which involves adding an aluminum salt to the aqueous solution, and thereafter increasing the pH of the solution to 3.5-4.5.
U.S. Pat. No. 4,094,739 describes a method for removing bacterial cells from an aqueous mixture containing a polysaccharide produced by bacterial fermentation, which method involves the steps of killing the said bacterial cells, and then causing the aqueous mixture to undergo a second fermentation with a Trichoderma sp. mold to effect solubilization of the said killed cells.
U.S. Pat. No. 4,135,979 describes a method for clarifying a xanthan gum fermentation broth which involves heating the broth and filtering the broth while maintaining it at a temperature of 112.degree.-160.degree. C., wherein the broth contains at least about 1.5 weight percent of xanthan gum.
In most of the prior art procedures involving clarification and purification treatment of bacterial fermentation broths, the resultant biosynthetic heteropolysaccharide product is recovered either as a soluble component of a very dilute solution, or in the form of a recovered solid precipitate. A dilute solution is a disadvantage because it cannot be shipped economically. A solid precipitate is a disadvantage because it must be redissolved for purposes of most applications such as secondary and tertiary petroleum recovery processes.
Accordingly, it is an object of this invention to provide an improved process for producing a clarified biosynthetic heteropolysaccharide fermentation broth.
It is another object of this invention to provide a process for producing a concentrated solution of Xanthomonas hydrophilic colloid.
It is a further object of this invention to provide a clarified Xanthomonas fermentation broth containing at least about 8 weight percent of dissolved xanthan gum, and having a viscosity between about 10,000-20,000 centipoises.
Other objects and advantages of the present invention shall become apparent from the accompanying description and example.